Unhardened, fogged emulsions and method of processing to positive images



United rates 3,062,651 Patented Nov. 6, 1952 fine Peter J. Hllson, Harrow, England, assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey N Drawing. Filed Jan. 21, 1959, Ser. No. 788,042

7 Claims. (Cl. 96-95) This application relates to photography, and more particularly, to that branch of photography adapted to the production of direct positive images.

It is known that ordinary photographic silver halide emulsions, and particularly unwashed silver chloride emulsions, can be treated with formalin in an aqueous solution to produce uniform fog, and that the fogged emulsion can be exposed to an original using minus blue illumination, e.g., yellow or red illumination, and then developed to a positive by bathing in a conventional blacloand-white developer. However, to obtain sufficient fogging by this method, it is necessary to use formalin having a high concentration of formaldehyde, for example. While such an expedient does increase fogging, it also causes hardening of the emulsion vehicle, usually gelatin, to such an extent that it is quite difficult to redisperse the fogged emulsion as is customary before coating on a suitable support. On the other hand, if optimum amounts of formaldehyde are present for re-dspersing purposes, then undesirably long fogging times (especially for single-solution types) are required.

Sometimes, instead of chemically fogging the emulsions as described above, the emulsions are given a short exposure (uniformly) to blue light and then a printing exposure (image-wise) to long wavelength radiation. Some of the effect of the blue exposure is destroyed so that the areas not exposed to the long-wavelength radiation become developable. This is known as the Herschel effect. In order to increase reversal speed and to increase the contrast of the final print, various chemical desensitizers have been used in the emulsions.

I have now found a new method of producing controlled fog in photographic silver halide emulsions, wherein the use of formaldehyde solutions can be entirely avoided.

It is, therefore, an object of my invention to provide fogged, photographic silver halide emulsions, which can be exposed and developed to a direct positive image. Another object is to provide a method of making such photographic silver halide emulsions. Still another object is to provide fogged photographic silver halide emulsions, which need not contain formaldehyde. Yet another object of my invention is to provide a method ofprocessing the fogged, photographic silver halide emulsions of my invention to produce positive images directly, without reversal re-exposure, or first processing a negative and then exposing a second photographic emulsion to obtain a positive. Other objects will become apparent from a consideration of the following description and examples.

I have found that photographic silver halide emulsions, and more particularly photographic gelatino-silver-chloride emulsions, can be fogged by treatment with thiourea dioxide, followed by treatment with a desensitizer, and then exposed in the normal way With minus blue illumination, e.g. red or yellow illumination, and developed to a positive image.

It appears that my invention depends upon the ability of thiourea dioxide to produce a controllable degree of fog so that on development after a minus blue exposure, the unexposed areas develop spontaneously but, in the exposed areas the fog centers will be removed and the silver halide will not develop. While the thiourea dioxide used in my invention is a known reducing agent, it was unexpected that this agent could be used in emulsions to produce controllable fog that could be adapted to produce direct images.

The thiourea dioxide used in my invention can advantageously be represented by either of the following two formulas:

O SOzH i Nn2 =NH NH2 ON}IZ The most advantageous amount of thiourea dioxide employed in the fogging treatment can be varied, depending upon the silver halide concentration, temperature of emulsion during fogging, etc. In general, I have found that from about 20 to 250 mg./mole of silver halide is quite adequate for the purposes of my invention.

The silver halide emulsion employed in my invention is generally an unwashed silver chloride emulsion, which means that the emulsion contains water-soluble salts, such as alkali metal or ammonium halides (eg. sodium chloride, potassium chloride, ammonium chloride, etc.) and nitrates. Other silver halides can also be used in my invention and include, for example, silver chlorobrornide, silver chloroiodide, etc.

The emulsion vehicle can be any hydrophilic colloid of the type customarily employed in photographic emulsions, such as gelatin, albumen, gum arabic, agar agar, polyvinyl alcohols (water-soluble or swellable), polyvinyl acetals (water-soluble or swellable), hydrolyzed cellulose esters or others, etc.

The fogging of the emulsions is effected, according to my invention, under alkaline conditions, e.g., a pH of at least about 8.0 and preferably 9.0 to 10.0, or higher. The emulsion is then held at an elevated temperature for several minutes until the desired degree of fogging is effected. In general, fogging can be completed in a short (10-20 minutes) period of time by holding the emulsions at an elevated temperature (e.g., 150 F.). Use of lower temperatures requires more extended fogging times. For example, the emulsions can be satisfactorily fogged in about 10 minutes at 125 F., but require 30 to 60 minutes at about F. I

The amount of fogging can be increased (i.e. speed of reversal decreased) by raising the pH, increasing the concentration of thiourea dioxide, raising the temperature or lengthening the time the emulsion is held at an elevated tampzrature.

l have found that especially useful results can be obtained if the thiourea dioxide is added to the emulsion under acidic conditions (i.e. pH of the emulsion is below 7.0, e.g., 5.0 or 4.0), and the pH of the emulsion is brought to the alkaline side of neutrality and fogged as described above. The pH of the emulsion can then be modified as desired (e.g., be made neutral), .and a desensitizer added. The emulsion is then ready for coating on a suitable support, such as paper, film (e.g., cellulose acetate, polystyrene, polyester, etc.), metal, etc.

The desensitizers which can be used in my invention are conventional in the art, and the operation of the invention is not dependent upon the use of any particular desensitizer, or class of desensitizers. Typical desensitizers include nitrobenzimidazoles, 6-nitrobenzotriazole, 4-nitro-6-chlorobenzotriazole, phenosafranine, pinakryptol green, 2 [(p dimethylaminophenylimino)methyl] quinoline ethiodide, nitrostyrylcyclammonium quaternary salts and bases (e.g., those of Kendall et al. US. Patents 2,669,515, issued February 16, 1954, and 2,541,472, issued February 13, 1951), etc.

Since the photographic emulsions of my invention are not hardened during fogging, they can be developed in a Example 1 To 3.6 kilograms of an ordinary gelatino-silver-chloride emulsion containing one gram mole of silver chloride and 7.5 mole percent of excess chloride ion (as sodium chloride), and having .a pH of 4.0, were added 12 cc. of a 0.5 percent aqueous solution of thiourea dioxide. The pH of the emulsion was raised to about 9.0 by addition of an aqueous solution of borax and sodium metaborate. The emulsion was then held at about 120 F. for 20 minutes, the pH was adjusted to about 7.0 by the addition of aqueous sulphuric acid, and 167 cc. of a 0.5 percent solution of 6-ethoxy-l-methyl-2-m nitrostyrylquinolinium toluene-p-sulphonate CH3 OSO2CsH4- a(D) added. The emulsion was then coated on paper and dried. The dried emulsion was then exposed to a positive image with yellow light (e.g. using a No. 12 or No. 15 Wratten filter). The exposed emulsion was developed for 30 seconds at 68 F. in a developer having the following composition (diluted 1 part to 2 parts of water):

G. N-methyl-p-aminophenol sulphate 3.1 Hydroquinone 12.0 Sodium sulphite, anhydrous 45.0 Sodium carbonate, desiccated 67.5 Potassium bromide 1.9

Water to make 1 liter.

An excellent, high-contrast positive image was produced. Example 2 Example 1 was repeated except that 2-(2:4-dinitrobenzylidene)-1-ethyl-1:Z-dihydroquinoline:

| O Hs N 02 was substituted for 6-ethoxy-l-methyl-Z-m-nitrostyrylquinolinium toluene-p-sulphonate.

Example 3 Example 1 was repeated except that 6-nitrobenzotriazole:

ning developer and then washing off the untanned gelatin with hot water.

The emulsion made in Example 1 was coated on plates. It was exposed as before and was developed in a tanning solution made by mixing equal volumes of the following solutions, A and B.

Solution A: G. Catechol 30.0 Sodium sulphite. anhydrous 2.0 Potassium bromide 7.5 Water to make 1 litre.

Solution B:

Sodium hydroxide 17.5

Water to make 1 litre.

After conventional fixing with sodium thiosulphate (no hardener added). the unhardened gelatin was washed off in hot water F), leaving anexcellent relief pos1- tive image.

As indicated above. the degree of fogging contributed by the thiourea dioxide can be varied, depending upon the effects desired and the amount added to the emulsions. etc. In general, the emulsions are fogged to at least about 50 percent of their maximum density. Ordinarily, the density will thus be at least about 1.0. when developed in a developer of the type shown in Example 1 above for the time and temperature indicated in that example.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

What I claim as my invention and desire secured by Letters Patent of the United States is: v

1. A direct positive photographic emulsion comprising a silver halide emulsion uniformly fogged to at least 50 percent its maximum density, but not lessthan 1.0, by the addition thereto of from 20 to 250 mg. of thiourea dioxide per mole of silver halide and containing an organic, heterocyclic desensitizer of silver halide.

2. A direct positive photographic emulsion comprising a silver chloride emulsion uniformly fogged to at least 50 percent its maximum density, but not less than 1.0, by the addition thereto of from 20 to 250 mg. of thiourea dioxide per mole of silver chloride and containing an organic, heterocyclic desensitizer of silver chloride.

3. A direct positive photographic emulsion comprising a silver chloride emulsion uniformly fogged to at least 50 percent its maximum density, but not less than 1.0, by the addition thereto of from 20 to 250 mg. of thiourea dioxide per mole of silver chloride and containing a nitrostyrylcyclammonium quaternary salt desensitizer of silver chloride.

4. A direct positive photographic emulsion comprising a strongly fogged silver chloride emulsion uniformly fogged to at least 50 percent its maximum density, but not less than 1.0, by the addition thereto of from 20 to 250 mg. of thiourea dioxide per mole of silver chloride and containing 6-ethoxy-l-methyl-Z-m-nitrostyryrqumolinium toluene p-sulphonate.

5. A direct positive photographic emulsion comprising a strongly fogged silver chloride emulsion uniformly fogged to at least 50 percent its maximum density, but not less than 1.0, by the addition thereto of from 20 to 250 mg. of thiourea dioxide per mole of silver chloride and containing 2-(2:4-dinitrobenzylidene)-l-ethyl-1:2-dihydroquinoline.

6. A direct positive photographic emulsion comprising a strongly fogged silver chloride emulsion containing excess chloride ion uniformly fogged to at least 50 percent its maximum density, but not less than 1.0, by the addition thereto of from 20 to 250 mg. of thiourea dioxide per mole of silver chloride and containing a benzotriazole desensitizer.

7. A direct positive photographic emulsion comprising a strongly fogged silver chloride emulsion uniformly fogged to at least 50 percent its maximum density, but

5 6 not less than 1.0, by the addition thereto of from 20 t 2,669,515 Kendall et a1. Feb. 16, 1954 250 mg. of thiourea dioxide per mole of silver chloride 2,983,610 Allen et al. May 9, 1961 and containing 6-nitrobenzoiriazole.

FOREIGN PATENTS References Cit d in the file of this patent 6 Belgium y UNITED STATES PATENTS 789,823 Great Britain Jan. 29, 1958 2,262,055 Sease et a1. Nov, 11, 1941 2,339,309 Weber et al Jan. 18, 1944 OTHER REFERENCES 2,376,297 Weber May 15, 1945 HautOt et al.: Science et Industries Photographiques, 2,541,472 Kendall et a1. Feb. 13, 1951 10 1957, 2 P g 

1. A DIRECT POSITIVE PHOTOGRAPHIC EMULSION COMPRISING A SILVER HALIDE EMULSION UNIFORMLY FOGGED TO AT LEAST 50 PERCENT ITS MAXIMUM DENSITY, BUT NOT LESS THAN 1.0, BY THE ADDITION THERETO OF FROM 20 TO 250 MG. OF THIOUREA DIOXIDE PER MOLE OF SILVER HALIDE AND CONTAINING AN ORGANIC, HETEROCYCLIC DESENSITIZER OF SILVER HALIDE. 